Eligibility RequirementsA purchaser must be a U.S. citizen, over 18 years old, and legally eligible to purchase a firearm. They must provide a copy of a U.S. birth certificate, passport, proof of naturalization, or any official government that shows proof of citizenship. A U.S. military ID can be used if E5 or above.
You think the requirements were stricter in 1963? There is no "membership" to "join" CMP.
He bought his weapons under an assumed name, Alex Hiddel. Bought through Klein’s sporting goods, Shipped by US Mail to a PO Box. Things were different back then, and you’d have to drill down why he thought that was necessary,